1. Field of the Invention
The present invention pertains to electrochemical devices such as solid oxide fuel cells (SOFCs) or ceramic fuel cells, particularly thin film solid oxide fuel cells (TFSOFCs). More particularly, a porous metallic anode and a thin film conducting oxide porous cathode are provided, along with methods for forming the electrodes and a thin film electrolyte.
2. Description of Related Art
Fuel cells are energy-converting devices that use an oxidizer (e.g. oxygen in air) to convert the chemical energy in fuel (e.g. hydrogen) into electricity. A SOFC (also called a xe2x80x9cceramic fuel cellxe2x80x9d) generally comprises a solid electrolyte layer with an oxidizer electrode (cathode) on one side of the electrolyte and a fuel electrode (anode) on the other side. The electrodes are required to be porous, or at least permeable to oxidizer at the cathode and fuel at the anode, while the electrolyte layer is required to be dense so as to prevent leakage of gas across the layer. A TFSOFC has a thin electrolyte layer, on the order of 1-10 micrometers thick, as described, for example, in U.S. Pat. No. 5,753,385. This reduces the ohmic resistance of the electrolyte and increases the power density of the fuel cell. Because of the low electrolyte resistance, the TFSOFC can operate at lower temperatures. This increases the reliability and allows wider choices of materials for TFSOFC applications. Using the TFSOFC design can also reduce materials costs and reduce the volume and mass of the fuel cell for a given power output.
U.S. Pat. No. 5,753,385 discloses physical and chemical deposition techniques to synthesize the basic components of a TFSOFC. In one technique, the electrodes are formed from ceramic powders sputter coated with an appropriate metal and sintered to a porous compact. The electrolyte is formed by reactive magnetron deposition. The electrolyte-electrode interface is formed by chemical vapor deposition of zirconia compounds onto the porous electrodes.
U.S. Pat. No. 5,656,387 discloses an improved nickel and yttrium-stabilized zirconia (YSZ) anode and a method for making by DC magnetron sputtering. The films were deposited on a surface of yttria-stabilized zirconia (YSZ).
U.S. Pat. No. 5,106,654 discloses a method for matching thermal coefficients of expansion in fuel cell or other electrochemical devices. A tubular configuration not employing thin films is described.
YSZ thin film fuel cells have generally been formed by depositing the YSZ on a substrate that is not crystallographically ordered. Therefore, the YSZ is not ordered and thicker layers must be deposited to form a layer impermeable to gas. Michibata et al (xe2x80x9cPreparation of Stabilized Zirconia Electrolyte Films by Vacuum Evaporation,xe2x80x9d Denki Kagaku, 58, No. 11 (1990) demonstrated growth of dense but not atomically ordered YSZ films on nickel foil. They also provided no mechanism for increasing gas permeability of the nickel foil and claimed very low maximum power output (7 mW/cm2) of a resulting fuel cell.
To make thin film solid oxide fuel cells more efficient and less expensive to fabricate, improved methods for forming the porous electrodes and the non-porous electrolyte used in such devices are needed. The electrolyte should be defect-free to avoid charge and gas leakage across the cell, and thin to provide lower electrical resistance at moderate temperatures. Interconnect layers to make possible stacking of cells should be provided.
A method for forming a thin film solid oxide fuel cell (TFSOFC) with a porous metallic anode and an oxidizer-permeable cathode on opposite surfaces of a dense electrolyte layer is provided. The electrolyte layer may have an ordered crystal structure.
The fabrication process uses a thin dense metallic material such as nickel foil as a substrate on which to grow the electrolyte. The nickel foil may be appropriately rolled or otherwise processed to produce an ordered crystal structure that allows the electrolyte layer, epitaxially grown on the nickel substrate, to be crystallographically ordered. The nickel foil is later used as the anode after it is made porous by lithographic patterning and etching by chemical or physical processes.
Thin film oxide deposition technologies such as pulsed laser deposition (PLD) or metal organic chemical vapor deposition (MOCVD) can be used for the deposition of the oxide electrolyte as well as for the conducting oxide cathode. PLD is an ideal vehicle to develop very thin films for TFSOFC applications, while MOCVD is good for large area thin film fabrication. Sputtering, evaporation sol-gel, metal organic deposition (MOD), electron-beam evaporation, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), or other oxide film deposition techniques can also be used. Because the substrate is dense and not porous, and in foil form, a dense electrolyte layer is easily deposited on it, and the difficulty of forming a dense, uniform electrolyte layer on a porous substrate is avoided. Also, because the solid metal substrate is used as a support, the electrolyte layer can be very thin. In addition, since the substrate is a continuous foil and can be made atomically ordered, electrolyte film with ordered crystal structure can be grown on appropriately prepared metallic foil substrates such as nickel.
Chemical or physical etching or a physical process such as laser drilling may be used to fabricate pores in the metallic substrate (which will become the anode) after deposition of the electrolyte. The cathode layer can be deposited on the opposite side of the electrolyte layer, either before or after etching or physical drilling of the metallic substrate. The cathode is usually a conducting oxide layer, which can be deposited by PLD, MOCVD or other suitable oxide film deposition technique, thus forming the TFSOFC. A mixed ionic and electronic conductor film between the anode and the electrolyte may be deposited to enhance the activity of the porous anode structure. Stacked cells may be epitaxially grown using a substrate having an atomically ordered surface.